Heterogeneous palladium catalyst constructed with cross-linked hyperbranched poly(phenylacetylene) as polymer support: A reusable highly active ppm-level catalyst for multiple cross-coupling reactions

Article abstract of DOI:10.1016/j.apcata.2014.10.009

We demonstrate in this work a unique strategy for the synthesis of heterogeneous Pd catalysts with the use of a hyperbranched poly(phenylacetylene) containing pendant alkyne groups as the cross-linkable polymer substrate. It utilizes the dual functions of Pd-based catalysts in catalyzing both alkyne polymerization/oligomerization and coupling reactions. In the synthesis, a homogeneous Pd(II) catalyst catalyzes the cross-linking of the hyperbranched polymer and simultaneously encapsulates itself into the crosslinked polymer matrix, rendering the heterogeneous catalyst at high yield and high percentage of Pd encapsulation. Three homogeneous catalysts having different ligands (triphenylphosphine, a diphosphine ligand, and a diimine ligand) have been examined for the cross-linking encapsulation and the resulting heterogeneous catalysts have been evaluated for their catalytic performance in coupling reactions. Among the various heterogeneous catalysts obtained, a triphenylphosphine-ligated catalyst, HBPPA-Pd-2, appears to be the optimum one. It shows high activity in catalyzing the Suzuki-Miyaura reactions, the Mizoroki-Heck reactions, and the allylic arylation reactions under air with the Pd loading at as low as mol ppm or even mol ppb levels relative to the reactants. Meanwhile, it facilitates the Suzuki-Miyaura reactions of challenging less reactive aryl chlorides/heteroaryl bromides and the Mizoroki-Heck reactions of aryl bromides as reactants. In addition, it behaves truly as a heterogeneous catalyst with high reusability and low catalyst leaching during the reactions.

Full text of DOI:10.1016/j.apcata.2014.10.009

Applied Catalysis A: General 489 (2015) 61–71
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heterogeneous palladium catalyst constructed with cross-linked
hyperbranched poly(phenylacetylene) as polymer support: A reusable
highly active ppm-level catalyst for multiple cross-coupling reactions
Zhongmin Dong, Zhibin Ye∗
Bharti School of Engineering, Laurentian University, Sudbury, Ontario P3E 2C6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
We demonstrate in this work a unique strategy for the synthesis of heterogeneous Pd catalysts with the
use of a hyperbranched poly(phenylacetylene) containing pendant alkyne groups as the cross-linkable
polymer substrate. It utilizes the dual functions of Pd-based catalysts in catalyzing both alkyne polymer-
ization/oligomerization and coupling reactions. In the synthesis, a homogeneous Pd(II) catalyst catalyzes
the cross-linking of the hyperbranched polymer and simultaneously encapsulates itself into the cross-
linked polymer matrix, rendering the heterogeneous catalyst at high yield and high percentage of Pd
encapsulation. Three homogeneous catalysts having different ligands (triphenylphosphine, a diphosphine
ligand, and a diimine ligand) have been examined for the cross-linking encapsulation and the result-
ing heterogeneous catalysts have been evaluated for their catalytic performance in coupling reactions.
Among the various heterogeneous catalysts obtained, a triphenylphosphine-ligated catalyst, HBPPA-Pd-
2, appears to be the optimum one. It shows high activity in catalyzing the Suzuki–Miyaura reactions, the
Mizoroki–Heck reactions, and the allylic arylation reactions under air with the Pd loading at as low as
mol ppm or even mol ppb levels relative to the reactants. Meanwhile, it facilitates the Suzuki–Miyaura
reactions of challenging less reactive aryl chlorides/heteroaryl bromides and the Mizoroki–Heck reac-
tions of aryl bromides as reactants. In addition, it behaves truly as a heterogeneous catalyst with high
reusability and low catalyst leaching during the reactions.
Received 15 July 2014
Received in revised form
29 September 2014
Accepted 5 October 2014
Available online 12 October 2014
Keywords:
Heterogeneous Pd catalyst
Cross-coupling reactions
Hyperbranched polymer
Suzuki–Miyaura reactions
Mizoroki–Heck reactions
Allylic arylation reactions
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
minimum/negligible catalyst leaching during reactions, and high
versatility with broad applicability to multiple types of coupling
Palladium-catalyzed carbon–carbon cross-coupling reac-
tions [1–3], including the notable Suzuki–Miyaura [4–7] and
Mizoroki–Heck reactions [8–13], have played an important role
in modern organic synthesis by enabling many valuable organic
transformations. Due to the high cost of Pd as well as environ-
mental and health issues, reusable heterogeneous Pd catalysts
have drawn increasing attention for the industrial applications
of these coupling reactions since homogeneous Pd catalysts are
difficult to recover and reuse. Numerous heterogeneous catalysts
have been developed with the use of various organic polymeric
and inorganic supports through different catalyst immobiliza-
tion/encapsulation strategies [14–22]. But the search is continuing
for high-performance heterogeneous catalysts of high activity with
Pd loading needed at as low as ppm levels, high reusability with
reactions and to difficult precursors such as aryl chlorides [23].
Such heterogeneous catalysts of combined performance properties
are still limited despite the enormous investigations in the area.
Relative to the inorganic supports, organic polymer supports
show the advantage in their convenient synthesis and the ver-
satility in tuning their composition/functionality for controllable
catalyst immobilization/encapsulation. A great variety of polymers,
including both soluble polymers [24–28] with controllable linear
[16,19,24–28] or dendritic/hyperbranched architectures [29–33]
and insoluble polymers [14–22], have been used as the substrate
for the immobilization/encapsulation of various Pd complexes
or Pd nanoparticles. With soluble polymer substrates, high cat-
alyst activity can be achieved due to their soluble nature, but
at the compromise of relatively higher catalyst leaching and the
requirement of special catalyst recovery methods such as biphasic
solvent systems and polymer precipitation with proper solvents
[24–28]. On the contrary, heterogeneous catalysts on insoluble
polymers offer much more convenient catalyst recovery and reuse
∗
Corresponding author. Tel.: +1 705 675 1151 2343; fax: +1 705 675 4862.
E-mail address: zye@laurentian.ca (Z. Ye).
http://dx.doi.org/10.1016/j.apcata.2014.10.009
0926-860X/© 2014 Elsevier B.V. All rights reserved.

62
Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
through simple procedures like filtration. In particular, insol-
uble cross-linked polymers show the further added advantage
of reduced catalyst leaching because the tightly cross-linked 3-
dimensional networks minimize the aggregation and leaching of
the encapsulated catalyst species. Due to these advantages, various
insoluble cross-linked polymers have been developed and used to
immobilize/encapsulate Pd nanoparticles and Pd complexes as het-
erogeneous catalysts [17,18,34]. They include broadly cross-linked
amphiphilic copolymer micelles [17,18,35–37], cross-linked poly-
methacrylates [38,39], cross-linked polystyrene-poly(ethylene
glycol) resins [40–42], cross-linked poly(amidoamine) dendrimers
[43], polyurea [44,45], self-assembled poly(imidazole) via binding
with Pd species [46,47], cross-linked hyperbranched polyethylenes
with self-supported Pd nanoparticles [48], cross-linked micro-
porous polymers [49–51], cross-linked reverse micelle [52],
cross-linked polystyrenes with different covalently tethered
functionalities [53–56], cross-linked ionic copolymers [57–59],
hydrogels [60,61], cross-linked chitosan [62,63], cross-linked
poly(1,3-diethynylbenzene) [64], etc.
The majority of heterogeneous Pd catalysts on cross-linked
polymer supports have been synthesized via stepwise procedures
comprised of the synthesis of the cross-linked polymers with
desired functionalities followed with catalyst immobilization. With
this synthetic strategy, the immobilized Pd catalysts are thus
bound either covalently or noncovalently on the internal surface
of the cross-linked polymer substrates instead of being incorpo-
rated within the polymer matrix. This increases the risk of catalyst
leaching. Only in some cases [35–37,41,43–48,52,62–64] the Pd
catalysts are embedded desirably within the cross-linked polymer
matrix wherein Pd encapsulation occurs during or prior to poly-
mer cross-linking in the synthesis. Meanwhile, the majority of the
heterogeneous catalysts on cross-linked polymer supports show
relatively low activity with a large Pd loading often required to facil-
itate the coupling reactions, and often have limited applicability to
only one or two types of couple reactions.
procedure [65]. HPLC-grade CH2Cl2 (99.5%, Fisher Scientific)
was deoxygenated and dried by using a solvent purification
system (Innovative Technology) before use. Methanol (ACS
˚
˚
reagent, Fisher Scientific) was dried with 3 A/5 A molecular
sieves before use. Deionized Water was obtained from a Barn-
stead/SynbronNanopure II water purification system. A palladium
atomic absorption standard solution, containing 1011 ppm of Pd in
5.1 wt% HCl, was purchased from Aldrich. N,N-dimethylformamide
(DMF, certified ACS grade), tetrahydrofuran (THF, certified grade),
hydrochloric acid (35–38 wt.% in water), KF (granular powder,
certified ACS grade) and K2CO3 (granular powder, certified ACS
grade) were received from Fisher Scientific and used without
further purification. Palladium (II) acetate (Pd(OAc)2, 98%) and
˛,˛’-bis(di-t-butylphosphino)-o-xylene (97%) were obtained
from Strem Chemicals and used as received. Mesoporous sil-
ica SBA-15 was purchased from Claytec Inc., and was vacuum
dried at 160 ◦C for 8 h before use. All other chemicals or sol-
vents, including 1-methyl-2-pyrrolidinone (NMP, ReagentPlus),
CsF (>99%), 1,3-diethynylbenzene (DEB, 96%), methanesulfonic
acid (MSA, 99.5%), triphenyl phosphine (PPh3, 99%), iodoben-
zene (PhI, 98%), 4’-iodoacetophenone (98%), bromobenzene
(PhBr, 99%), 1-bromo-4-nitrobenzene (99%), 4-bromobenzonitrile
(99%), 4’-bromoacetophenone (98%), 4-bromoanisole (99%), 4-
bromotoluene (98%), 2-bromotoluene (99%), 3-bromotoluene
(98%),
2-bromomesitylene
(98%),
1-bromonaphthalene
(97%), 2-bromopyridine (99%), 3-bromothianaphthene (95%),
chlorobenzene (PhCl, ReagentPlus), 4’-chloroacetophenone
(97%), 4-chlorotoluene (98%), 4-chloroanisole (99%), 4-
chlorobenzonitrile (99%), 4-methoxyphenylboronic acid (≥95%),
4-acetylphenylboronic acid, n-butyl acrylate (BA, >99%), styrene
(99%), triethyl amine (Et3N, 99%), phenylboronic acid (PBA,
>97%), cinnamyl acetate (99%), prenyl acetate (≥98%), sodium
tetraphenylborate (NaBPh4, ACS Reagent), nitric acid (HNO3,
70%), and hydrogen peroxide (H2O2, 50 wt.% in water), were all
purchased from Aldrich and used as received.
In this article, we report
a convenient alternative one-
step synthetic strategy that leads to a reusable, high-activity,
high-versatility heterogeneous catalyst with Pd species encapsu-
lated within a cross-linked hyperbranched poly(phenylacetylene)
(HBPPA) matrix. Different from all other previously reported syn-
theses, a triphenylphosphine-ligated cationic Pd catalyst is herein
used as an alkyne polymerization catalyst to cross-link the hyper-
branched poly(phenylacetylene) having pendant alkyne groups.
During the cross-linking, the Pd catalytic species are simulta-
neously embedded within the formed polymer networks, turning
themselves uniquely from a homogeneous polymerization catalyst
into a heterogeneous cross-coupling catalyst. The heterogeneous
catalyst synthesized herein has been found to efficiently catalyze
the Suzuki–Miyaura, Mizoroki–Heck, and allylic arylation reactions
at the Pd loadings of as low as ppm levels relative to reactants,
as well as good reusability with low catalyst leaching during the
reactions.
2.2. Measurements and characterizations
The characterization of HBPPA with gel permeation chromatog-
raphy (GPC) was carried out on a Polymer Laboratories PL-GPC220
system equipped with a triple-detection array comprising of a
differential refractive index (DRI) detector (from Polymer Laborato-
ries), a three-angle (45, 90, and 135◦) light scattering (LS) detector
(from Wyatt Technology) at a laser wavelength of 687 nm, and
a four-bridge capillary viscosity detector (from Polymer Labora-
tories). Details in the characterization have been reported in our
earlier paper [66].
All measurements with proton nuclear magnetic resonance
(1H NMR) spectroscopy were carried out on a Bruker AV500
spectrometer (500 MHz) at ambient temperature with CDCl3 as
solvent. Atomic absorption (AA) spectroscopy was performed on a
Perkin Elmer Precisely AAnalyst 400 spectrometer (Perkin Elmer)
equipped with a Pd element lamp (max. 30 mA, Perkin Elmer). The
data was collected with a WinLab32 software (Perkin Elmer). The
blank solution for the preparation of the standard Pd solutions and
sample dilution was composed of 11.1 vol% THF, 6.7 vol% HCl, and
3.4 vol% H2O2 in deionized water. For all the analyses, a calibration
curve was first established with Pd standard solutions with [Pd]
in the range of 0.25–10 mg/L. X-ray photoelectron spectroscopy
(XPS) measurements of the heterogeneous Pd catalysts were
carried out on a Thermo Scientific Theta Probe XPS spectrometer
(ThermoFisher). A monochromatic Al K␣ X-ray source was used,
with a spot area 400 ␮m. The samples were run in a standard
mode, i.e., all angle collected (60◦ angular acceptance) for the
survey spectra, and for the region spectra. Transition electron
2. Experimental
2.1. Materials
All manipulations and reactions were performed under atmo-
spheric conditions unless otherwise noted. In order to avoid the
influence of any residual Pd species absorbed on the surface,
all the glassware and magnetic stirrers used for cross-coupling
reactions were pre-treated with aqua regia, then washed with
distilled water before being dried. The acetonitrile Pd-diimine
-
catalyst, [(ArN C(Me)–(Me)C NAr)Pd(CH3) (N CMe)]+SbF6
(Ar = 2,6-(iPr)2C6H3), was synthesized according to a literature

Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
63
Table 1
Heterogeneous Pd catalysts by Pd-catalyzed cross-linking of HBPPA.a
c
Heterogeneous Pd catalyst
Catalyst for cross-linking
[alkyne]/[Pd]
yield%b
Pdenc
%
Pd%c
HBPPA-Pd-1
HBPPA-Pd-2
HBPPA-Pd-3
HBPPA-Pd-4
HBPPA-Pd-5
L1-Pd
L1-Pd
L1-Pd
L2-Pd
L3-Pd
4.4
8.8
26
8.8
8.8
93
96
98
77
88
75
97
97
68
92
2.6
1.9
0.6
1.7
1.9
a
Other conditions: [alkyne] = 0.14 M (HBPPA: 0.52 g), CH2Cl2/MeOH (v/v = 10:1) as solvent (5.5 mL), reaction at room temperature. L1-Pd:Pd(OAc)2/triphenylphosphine
(L1) at the [Pd]/[L1] ratio of 1: 6; L2-Pd: Pd(OAc)2/diphosphine (L2) at the [Pd]:[L2] ratio of = 1: 3; L3-Pd: acetonitrile Pd–diimine catalyst.
b
Yield of the heterogeneous catalyst calculated on the basis of weight relative to HBPPA.
Percentage of encapsulated Pd relative to the feed Pd(Pdenc%) and weight content of Pd in the heterogeneous catalysts (Pd%) were determined via atom absorption analysis
c
of each heterogeneous catalyst.
microscopy (TEM) images were collected on a JEOL 2010F field
emission electron microscope operated at 200 keV.
2.5. Synthesis of Si-ArI for 3-phase tests
A similar procedure used by Jones et al. [67] was employed
for the synthesis of heterogeneous mesoporous silica-supported
aryl iodide, Si-ArI. An aryl iodide-containing trimethoxysi-
lane compound (1 in Scheme 2) was first synthesized by
reacting 4-iodoacetophenone (1.35 g, 5.5 mmol) and 3-
aminopropyltrimethoxylsilane (0.895 g, 5 mmol) in 20 mL dry
MeOH under reflux at 75 ◦C for 20 h. After being cooled to room
temperature, MeOH was evaporated under reduced pressure
and the product was extracted with CH2Cl2. The mixture was
then concentrated and washed with cold hexane to afford 1.
Yield: 1.87 g (92%).1H NMR (CDCl3): 0.78 (m, 2H, –CH2Si), 1.84
[m, 4H,–(CH2)2CH2Si], 2.07 [s, 3H, N C(Me)], 3,34–3.52 [m, 9H,
Si(OMe)3], 7.44–7.63, (m, 4H, ArH).
Subsequently, 1 (1.63 g, 4 mmol) was added into a toluene
(20 mL) suspension of dry mesoporous silica SBA-15 (1.0 g) and the
mixture was stirred at 70 ◦C for 24 h under N2 protection. After
cooling to room temperature, the mixture was filtered and the solid
was washed with CH2Cl2 then hexane. After being dried under high
vacuum at 80 ◦C overnight, the silica-supported aryl iodide (Si-ArI)
was obtained as white powder. Yield: 1.258 g. Aryl iodine content
(based on TGA analysis): 22 wt%.
2.3. Synthesis of hyperbranched polyphenylacetylene (HBPPA)
The HBPPA containing pendant alkyne groups was synthesized
according to our earlier paper [66]. Under N2 protection, Pd(OAc)2
(7.1 mg, 32 ␮mol), ˛,˛’-bis(di-t-butylphosphino)-o-xylene (38 mg,
96 ␮mol), and methanol (8 mL) were mixed in a sealed 20 mL
glass vial and sonicated at room temperature until all the solids
were dissolved to form the catalyst solution. In the mean-
time, a flame-dried Schlenk flask was charged with PA (1.02 g,
10 mmol), DEB (0.374 g, 3 mmol), one drop of methanesulfonic
acid (MSA, ca. 0.05 mL), and CH2Cl2 (30 mL) under nitrogen pro-
tection. The flask was shielded with aluminum foil before the
catalyst solution was injected. The mixture was stirred under N2
atmosphere for 3 h at room temperature. At the completion of
the polymerization, the reaction mixture was poured into 2%-
acidified methanol (ca. 300 mL) to terminate the polymerization
and precipitate out the polymer. The orange polymer precipitate
underwent a redissolution–precipitation procedure for 3 cycles
to remove the catalyst and monomer residues. Finally, the poly-
mer was dried under vacuum at room temperature for 24 h and
then weighed (0.84 g, yield: 60%). Triple-detection GPC charac-
terization: number-average molecular weight, Mn = 27.2 kg/mol;
polydispersity index, PDI = 1.57 (both measured with light scat-
tering detector); Mark–Houwink exponent, ˛ = 0.20. 1H NMR
characterization: DEB content in the copolymer, 23 mol%; per-
centage of dual-inserted DEB, 29%; content of the pendent alkyne
groups, 41 alkyne group per chain or 1.5 mmol/g; branching den-
sity, 6.7 mol% (number of dual-inserted DEB/the number of all
inserted monomer units × 100%).
2.6. Mizoroki–Heck reactions catalyzed with heterogeneous Pd
catalysts
A typical procedure (for run H7 in Table 2) for Mizoroki–Heck
reactions of different aryl halides is as follows. The heterogeneous
catalyst HBPPA-Pd-2 (0.2 mg, containing 0.036 ␮mol of Pd), 4ꢀ-
bromoacetophenone (72 mg, 0.36 mmol), n-butyl acrylate (60 mg,
0.47 mmol), K2CO3 (75 mg, 0.54 mmol), and DMF (0.6 mL) were
combined in a clean test tube with a magnetic stirrer. The sealed test
tube was immersed into an oil bath set at 130 ◦C and was stirred.
The conversion of the reaction was monitored with 1H NMR spec-
troscopy. At the completion of the reaction, the insoluble salts and
heterogeneous catalyst were separated from the organic solution
by filtration. Water was used to wash off the inorganic salts and the
catalyst was recovered.
2.4. Synthesis of heterogeneous Pd catalysts with cross-linked
HBPPA as polymer support
A typical synthetic procedure (for HBPPA-Pd-2 in Table 1) is as
follows. HBPPA (0.52 g, containing 0.9 mmol alkyne groups), PPh3
(0.142 g, 0.54 mmol), Pd(OAc)2 (20 mg, 0.09 mmol), CH2Cl2 (5 mL)
and MeOH (0.5 mL) were combined in a test tube under N2 pro-
tection. Then one drop of MSA (ca. 0.05 mL) was added into the
system to start the cross-linking reaction. The mixture was stirred
at room temperature overnight. Visible brown insoluble precipi-
tates formed gradually during the reaction. At the completion of the
reaction, the mixture was filtrated and the insoluble cross-linked
polymer gels were washed with excessive DMF then THF to remove
any residual soluble catalyst and ligand before being dried at 60 ◦C
overnight. Yield: 0.5506 g; percentage of Pd encapsulation: 90.8%;
Pd content in the heterogeneous catalyst: 1.9 wt.% or 0.18 mmol
Pd/g of catalyst.
2.7. Suzuki–Miyaura reactions catalyzed with HBPPA-Pd-2
A
typical procedure (for run S2 in Table 3) for the
Suzuki–Miyaura reactions of different aryl bromides is as follows.
HBPPA-Pd-2 (0.06 mg, contain 0.011 ␮mol of Pd), PhBr (1.70 g,
11 mmol), PBA (1.58 g, 13 mmol), K2CO3 (2.24 g, 16 mmol), and
water (18 mL) were combined in a clean test tube with a mag-
netic stirrer. The test tube was sealed and placed in an oil base
set at 100 ◦C and was stirred until the reaction reached comple-
tion. During the reaction, the conversion of PhBr was monitored by

64
Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
Table 2
Mizoroki–Heck reactions carried out with the various heterogeneous catalysts.a
Run
Cat.
R1
X
R2
Pd%b (mol ppm)
t (h)
conv.c (%)
TON (×103)
TOF (×103 h−1
)
H1
H2
H3
H4
H5
H6
H7
H8
HBPPA-Pd-1
HBPPA-Pd-2
HBPPA-Pd-3
HBPPA-Pd-4
HBPPA-Pd-5
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
HBPPA-Pd-2
H
H
H
H
H
H
COMe
COMe
COMe
CN
H
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
St
CO2Bu
CO2Bu
CO2Bu
CO2Bu
100
100
100
100
100
10
5
4
6
>99
>99
>99
94
>99
86
>99
93
94
10
10
10
9.4
10
86
10
19
9.4
10
10
2.3
1.7
2
2.5
1.7
0.4
0.5
1.8
0.5
0.8
0.4
0.5
0.4
0.1
0.07
24
20
48
20
24
24
20
24
24
24
100
50
H9
100
100
100
100
100
H10
H11
H12
H13
>99
>99
23
Me
OMe
17
a
Other reaction conditions: [ArX]:[alkene]:[base] = 1:1.3:1.5; [ArX] = 0.6 mmol/mL, DMF (4 mL) as the solvent for all reactions; in the reactions between PhI and BA (H1–H6),
Et3N was the base and reaction temperature was 100 ◦C; in the reactions (H7–H13) with aryl bomides, K2CO3 was the base and reaction temperature was 130 ◦C.
b
Pd molar loading relative to ArX.
c
Conversion of ArX determined with 1H NMR spectroscopy.
1H NMR spectroscopy. At the completion of the reaction, the mix-
ture was cooled down to room temperature and washed with ethyl
acetate 3 times (2 mL × 3). The insoluble heterogeneous catalyst
was recovered by filtration.
As for the Suzuki–Miyaura reactions with heteroaryl halides,
the following is a typical procedure (for S16 in Table 3). 2-
Bromopyridine (0.095 g, 0.6 mmol), PBA (0.088 g, 0.72 mmol),
HBPPA-Pd-2 (16.5 mg, contain 3 ␮mol of Pd), K3PO4 (0.191 g,
0.9 mmol), and water/ethanol mixture (v/v = 2:1, 1 mL) were
combined in a clean test tube having a magnetic stirring bar. The
test tube was sealed and placed in an oil bath set at 100 ◦C. At
the completion of the reaction, the mixture was cooled to room
temperature and washed with ethyl acetate/water 3 times to
extract the organic product.
A
typical procedure (for run S18 in Table 3) for the
Suzuki–Miyaura reactions of PBA with various aryl chlorides
is as follows. HBPPA-Pd-2 (3.3 mg, contain 0.6 ␮mol of Pd), PBA
(0.088 g, 0.72 mmol), CsF (0.137 g, 0.9 mmol), and water (1.5 mL)
Table 3
Suzuki–Miyaura reactions catalyzed with HBPPA-Pd-2.a
Run
R1
X
R2
Pd%b (mol ppm)
t (h)
conv.c (%)
TON (×103)
TOF (×103 h−1
)
S1
S2
S3
S4
S5
S6
S7
S8
H
H
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
H
H
OMe
COMe
OMe
COMe
H
10
1
0.3
10
10
10
10
10
10
50
50
10
10
10
10
5000
5000
1000
1000
1000
1000
1000
4
10
17
1
1
8
8
10
8
8
24
4
4
6
8
24
20
8
>99
>99
96
>99
>99
>99
>99
90
100
1000
3200
100
100
100
100
90
95
47
43
100
100
88
100
0.19
0.2
1
25
100
187
100
100
13
13
9
12
5.8
4-COMe
4-CN
4-Me
3-Me
2-Me
4-OMe
S9
95
93
85
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
1-bromonaphthalened
2-bromomesitylenee
1.8
25
25
15
H
H
4-Me
4-Me
>99
>99
88
>99
96
>99
>99
>99
>99
>99
>99
13
2-bromopyridinef
0.01
0.01
0.13
0.13
0.05
0.04
0.04
2-bromothianaphtheneg
H
H
H
H
H
H
4-COMe
CN
H
4-Me
4-OMe
8
1
1
1
1
20
24
28
a
Other reaction conditions: [ArX]:[R2-PBA]:[base] = 1: 1.3: 1.5; T = 100 ◦C; in the reactions with ArBrs (S1–S15), [ArBr] = 0.6 mmol/mL, K2CO3 was the base and water was
the solvent; in the reactions with heteroaryl bromides (S16 and S17), [ArBr] = 0.6 mmol/mL, K3PO4 was the base and water/ethanol (v/v = 2:1) was the solvent; in the reactions
with ArCls (S18–22), [ArCl] = 0.4 mol/L, CsF was the base, water was the solvent along with TBAB as phase transfer agent at [TBAB]:[ArX] = 1:1.
b
Pd molar loading relative to ArX.
c
Conversion of ArX determined with 1H NMR spectroscopy.
d
1-Bromonaphthalene was used as the reactant.
2-Bromomesitylene was used as the reactant.
2-Bromopyridine was used as the reactant.
2-Bromothianaphthene was used as the reactant.
e
f
g

Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
65
were combined in a clean test tube with a magnetic stirrer. The
mixture was heated in an oil bath set at 100 ◦C until the solids were
dissolved. Then 4ꢀ-chloroacetophenone (93 mg, 0.6 mmol) and
tetra-n-butylammonium bromide (0.19 g, 0.6 mmol) were added
to the system. The test tube was sealed and placed in the oil base at
100 ◦C. The reaction was monitored by 1H NMR spectroscopy. At the
completion of the reaction, the mixture was cooled down to room
temperatureand diluted with ethyl acetate (1 mL) and water (4 mL).
The insoluble heterogeneous catalyst was recovered by filtration.
under exactly the same conditions. Each of two test tubes was
charged with HBPPA-Pd-2 (16.5 mg, containing 3 ␮mol of Pd), PhBr
(0.47 g, 3 mmol), PBA (0.44 g, 3.6 mmol), K2CO3 (0.62 g, 4.5 mmol),
and water (5 mL). The two test tubes were then sealed and heated
at 100 ◦C to start the reaction. The conversion of both reactions
was monitored with 1H NMR spectroscopy. The control run did
not involve hot filtration and was terminated after PhBr conversion
reached 100%. For the other run involving hot filtration, the reac-
tion mixture was quickly filtered with a syringe filter after 0.5 h of
reaction (PhBr conversion: 71%) to remove the heterogeneous cat-
alyst. The filtrate was immediately injected into another clean test
tube, which was stirred at 100 ◦C for 40 h with the conversion of
PhBr monitored with 1H NMR spectroscopy.
2.8. Allylic arylation reactions catalyzed with HBPPA-Pd-2
A typical procedure (for run A4 in Table 4) for the allylic ary-
lation reactions is as follows. HBPPA-Pd-2 (0.15 mg, containing
0.027 ␮mol of Pd), cinnamyl acetate (95.2 mg, 0.54 mmol), PBA
(85.5 mg, 0.7 mmol), KF (47.1 mg, 0.81 mmol), DMF (0.5 mL) and
water (0.5 mL) were combined in a clean test tube with a magnetic
stirrer. The sealed test tube was placed in an oil bath set at 100 ◦C
and was stirred for a predetermined reaction time. The conver-
sion of acetate was monitored with 1H NMR spectroscopy. After the
reaction, the heterogeneous catalyst was recovered by filtration.
2.11. 3-Phase tests
3-Phase tests using immobilized aryl halide Si-ArI were per-
formed for Mizoroki–Heck reaction. All the liquid reagents and
˚
solvents involved were dried with 4 A molecular sieves before
use to avoid possible hydrolysis of Si-ArI. In the typical reaction,
4-iodoacetophenone (0.295 g, 1.2 mmol), n-butyl acrylate (0.2 g,
1.6 mmol), Et3N (0.18 g, 1.8 mmol), Si-ArI (0.15 g, containing ca.
0.12 mmol ArI), HBPPA-Pd-2 (6.7 mg, containing 1.2 ␮mol of Pd)
and DMF (2 mL) were combined in a clean test tube having a mag-
netic stirring bar. The test tube was sealed and placed in an oil bath
set at 100 ◦C. After a pre-determined reaction time, the catalyst was
picked out and the insoluble Si-ArR was recovered by filtration and
washed with large amount of dry DMF/THF to remove all the sol-
uble small molecules. Subsequently, Si-ArR was hydrolyzed with
1 mol/L HCl solution overnight and extracted by CH2Cl2 (2 mL × 3
times). The solvent was then removed and the resulting compound
2 by hydrolysis (see Scheme 2) was characterized by NMR.
2.9. Reusability test
The reusability of HBPPA-Pd-2 in the Suzuki–Miyaura reaction
between PhBr and PBA was carried out at the higher Pd loading of
0.1 mol% relative to PhBr. The procedure is as follows. HBPPA-Pd-
2 (16.5 mg, containing 3 ␮mol of Pd), PhBr (0.47 g, 3 mmol), PBA
(0.44 g, 3.6 mmol), K2CO3 (0.62 g, 4.5 mmol) and water (5 mL) were
combined in a clean test tube with a magnetic stirrer. The tube was
then sealed and the mixture was stirred at 100 ◦C for 3 h. The con-
version of PhBr was determined with 1H NMR spectroscopy. At the
completion of the reaction, the heterogeneous catalyst was sepa-
rated from the reaction mixture by filtration. While the filtrate was
characterized with atomic absorption spectroscopy, the heteroge-
neous catalyst was washed with acetone for 4 times (10 mL × 4) and
was used to catalyze the next cycle of reaction following drying.
3. Results and discussion
3.1. Synthesis of heterogeneous Pd catalysts by catalytic
cross-linking of HBPPA
2.10. Hot filtration test
Our concept for the synthesis of the heterogeneous Pd cat-
alysts benefits from the unique dual capability of Pd-based
catalysts in catalyzing both alkyne polymerization/oligomerization
and cross-coupling reactions [64]. We employ a hyperbranched
In the hot filtration test, two Suzuki–Miyaura reactions (a con-
trol run and a hot filtration run) were simultaneously carried out
Table 4
Allylic arylation reactions catalyzed with HBPPA-Pd-2a.
Run
A
B
Pd% (mol ppm)
C
Time (h)
Conv.b (%)
TON (×103)
TOF (×103 h−1
)
A1
A2
A3
50
10
1
6
24
48
>99
>99
64
20
100
640
3.3
4.2
13
A4
A5
50
10
8
24
>99
>99
20
100
2.5
4.2
A6
A7
50
50
24
24
>99
90
20
18
0.83
0.75
a
Other reaction conditions: for all reactions, the solvent was water/DMF mixture (v/v = 1:1; total 1 mL); T = 100 ◦C; [allylic acetate (A)] = 0.6 mmol/mL; for reactions with
NaBPh4, [A]:[NaBPh4] = 1:1.3; for reactions with PBA, KF was used as base, [A]:[PBA]:[KF] = 1:1.3:1.5.
b
Conversion of allylic acetate determined with 1H NMR spectroscopy.

66
Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
Fig. 1. TEM image of HBPPA-Pd-2 (a) and XPS Pd 3d spectra of HBPPA-Pd-1 and HBPPA-Pd-2 (b).
poly(phenylacetylene) (HBPPA) containing pendant alkyne groups
(at a content of 1.5 mmol/g) as the cross-linkable polymer sub-
strate to construct the heterogeneous Pd catalysts by simultaneous
Pd-catalyzed cross-linking and encapsulation of the Pd species.
This HBPPA was synthesized by copolymerization of pheny-
lacetylene and 1,3-diethynylbenzene with the use of an in situ
generated cationic Pd(II) catalyst system, Pd(OAc)2/␣,␣ꢀ-bis(di-
tert-butylphosphino)-o-xylene/methanesulfonic acid (MSA), a syn-
thetic methodology developed in our earlier work [66]. The
cross-linking of HBPPA is achieved herein through the polymer-
ization/oligomerization of its pendant alkyne groups with the
homogeneous cationic Pd(II) catalyst to be encapsulated [64]. Dur-
ing the cross-linking, the homogeneous Pd(II) catalyst, along with
its ligand, is simultaneously encapsulated into the cross-linked
polymer network via physical entrapment and/or coordinative
binding to the pendant alkyne groups, turning itself successfully
into supported ligated Pd catalyst for coupling reactions (see
Scheme 1). In this way, the structure of the polymer matrix is better
controlled and different ligated Pd complexes can be employed.
The catalytic performance of the ligated Pd catalysts in
cross-coupling reactions is heavily related to their ligands [23].
Considering this, we have chosen to use three different homo-
geneous Pd(II) catalysts with varying ligands, triphenylphosphine
(L1), ␣,␣ꢀ-bis(di-tert-butylphosphino)-o-xylene (L2) as a diphos-
of the ligand on catalytic performance of the resulting hetero-
geneous catalysts and to obtain high-performance catalysts. The
two phosphine-ligated catalysts (L1-Pd and L2-Pd) were in situ
generated by mixing Pd(OAc)2 with the corresponding ligands
(L1 or L2) in the presence of a catalytic amount of methanesul-
fonic acid (MSA), which generates cationic Pd(II) centers [66]. A
cationic acetonitrile Pd–diimine complex (L3-Pd), a well-known
ethylene polymerization catalyst [65,68–78] also shown to effec-
tively oligomerize alkynes [79], was used as the single-component
diimine-ligated catalyst. While the diphosphine-ligated cata-
lyst L2-Pd has been well demonstrated to polymerize alkynes
[66,80,81], we have found that the monophosphine-ligated cata-
lyst L1-Pd is also highly efficient in facilitating the polymerization of
phenylacetylene and 1,3-diethynylbenzene. In the case with L1-Pd,
the cross-linking of HBPPA was carried out at different [alkyne]/[Pd]
feed ratios (4.4:1, 8.8:1, and 26:1) while the ratio of 8.8:1 was used
in those with L2-Pd and L3-Pd.
The simultaneous cross-linking and catalyst encapsulation were
accomplished conveniently by simply mixing and reacting HBPPA
with the corresponding catalyst in solution. In all cases, success-
ful cross-linking occurred with the gelled polymer precipitated
out during the course of the reaction. At the completion of each
reaction, the precipitate was thoroughly washed to remove pos-
sible non-cross-linked polymer as well as non-encapsulated Pd
and ligand species, and was subsequently dried, rendering the cor-
responding heterogeneous supported Pd catalyst as dark or light
brown particles. Table 1 lists the various heterogeneous catalysts
synthesized with this strategy, along with their characterization
results including yield of supported catalyst, Pd content, and per-
centage of Pd encapsulation.
phine ligand, and
a bidentate diimine ligand (L3), for the
cross-linking reaction and encapsulation so as to study the effect
Generally, the heterogeneous catalyst was obtained at high yield
(ca. 90%) in each case except that with L2-Pd possibly because of
its lower activity. With L1-Pd, increasing the [alkyne]/[Pd] feed
ratio by decreasing the amount of the Pd catalyst at the fixed
[alkyne] = 0.14 M leads to the increase in both yield (from 93 to 98%)
and percentage of Pd encapsulation (from 75 to 97%). This indicates
that the cross-linking was less efficient at the low [alkyne]/[Pd]
feed ratio of 4.4 since the catalyst was at the alkyne-starved
state. Comparing the three heterogeneous catalysts (HBPPA-Pd-2,
HBPPA-Pd-4, and HBPPA-Pd-5) obtained at the same [alkyne]/[Pd]
ratio, the percentage of Pd encapsulation increases in the order
HBPPA-Pd-4 < HBPPA-Pd-5 < HBPPA-Pd-2, indicating that L1-Pd is
the optimum one for quantitative encapsulation. By observation
with optical microscopy, the resulting heterogeneous catalysts all
have an average particle size of around 0.7 mm except HBPPA-Pd-2
whose average size is around 0.4 mm.
Scheme 1. Synthesis of heterogeneous Pd catalysts (HBPPA-Pd) containing different
Pd complexes by Pd-catalyzed cross-linking of HBPPA.

Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
67
effective than the diphosphine and diimine ligands used herein for
effecting the coupling reaction.
100
80
60
40
20
0
HBPPA-Pd-3
HBPPA-Pd-2
Among the three PPh3-ligated heterogeneous catalysts (HBPPA-
Pd-1 to HBPPA-Pd-3), HBPPA-Pd-3 having the lowest Pd content is
least active with the presence of an initial induction period of 2 h
in the reaction. This is reasoned to result from its highest polymer
content. Since the reactants have to diffuse through the polymer
matrix to reach the catalytic Pd species for the reactions, increas-
ing polymer content is expected to slow down the reaction due to
enhanced diffusion resistance. In particular, HBPPA-1 and HBPPA-2
show similar conversion curves with HBPPA-2 being slightly more
active. From this set of runs, we can conclude that PPh3-ligated
HBPPA-2 is the most optimum catalyst among the five heteroge-
neous catalysts given its catalytic performance and high efficiency
in Pd encapsulation. All subsequent investigations have thus been
carried out exclusively with the use of HBPPA-2.
HBPPA-Pd-4
HBPPA-Pd-5
HBPPA-Pd-1
20
0
10
30
time (h)
Fig. 2. Mizoroki–Heck reaction between PhI and BA catalyzed with
various heterogeneous catalysts. Reaction conditions: [PhI] = 0.6 mol/L,
[PhI]:[BA]:[Et3N] = 1:1.3:1.5, Pd loading relative to PhI = 100 mol ppm, DMF as
solvent at 100 ◦C.
Runs H6–H13 in Table 2 are other Mizoroki–Heck reactions
carried out with HBPPA-Pd-2 using PhI and aryl bromides as
the reactant. The highest TON (86,000) and TOF (1800 h−1) were
achieved in the reaction between PhI and BA (run H6) at the
lowest Pd loading of 10 mol ppm. Up to date, the highest TON
(2 × 106) and TOF (40 × 103 h−1) values of Mizoroki–Heck reac-
tion were recently achieved by Yamada et al. with a Si-nanowire
array-stabilized Pd-nanoparticle catalyst [89]. Although lower than
these values, the TON and TOF values we achieved are still among
the highest reported for the Mizoroki–Heck reactions carried out
with the use of heterogeneous Pd catalysts under similar condi-
tions [14,15,64]. In the case with relatively less active aryl bromides
as the reactant, the reactions were carried out at 130 ◦C with
K2CO3 as the base. Quantitative or nearly quantitative conver-
sions were achieved in the reactions (runs H7–11) of phenyl
bromide (PhBr) or activated aryl bromides (4ꢀ-bromoacetophenone
and 4-bromobenzonitrile) with BA or styrene at the Pd loading
of 100 mol ppm. With deactivated aryl bromides (4-bromotoluene
and 4-bromoanisole), the reactions (runs H12 and H13) were,
however, much slower at the same condition. In the reactions of
aryl bromides, the highest TON (19,000) and TOF (800 h−1) were
achieved with 4ꢀ-bromoacetophenone and BA as reactants at the
Pd loading of 50 mol ppm. All the products from the Mizoroki–Heck
reactions were featured with high E-selectivity with no/negligible
Z-isomers observed. These results confirm that HBPPA-Pd-2 is
highly effective in catalyzing the Mizoroki–Heck reactions.
The catalysts were further characterized with TEM and XPS. No
Pd(0) nanoparticles were observed in HBPPA-Pd-2 under TEM, as
shown in Fig. 1(a). Meanwhile, Fig. 1(b) shows the XPS Pd 3d spectra
for HBPPA-Pd-1 and HBPPA-Pd-2. Given the encapsulation of the Pd
species within the cross-linked polymer matrices at low contents,
the XPS peaks are generally very weak despite high numbers of
scans applied. For both HBPPA-Pd-1 and HBPPA-Pd-2, their Pd 3d3/2
and Pd 3d5/2 peaks center at around 343 and 338 eV, respectively.
This reveals that the encapsulated Pd species are primarily Pd(II)
rather than Pd(0) since Pd(II) typically has higher 3d binding ener-
gies (3d5/2 peak typically in the range of 336–339 eV) than the Pd(0)
(3d5/2 peak typically in the range of 334.6–335.7 eV) [64,82]. Mean-
while, it also indicates the absence of the reduction of Pd(II) species
during the polymerization and postpolymerization procedures.
3.2. Mizoroki–Heck reactions with various heterogeneous
catalysts
The five heterogeneous Pd catalysts were first evaluated and
compared for their performance in catalyzing the Mizoroki–Heck
reaction between iodobenzene (PhI) and n-butyl acrylate (BA) at
the Pd loading of 100 mol ppm relative to PhI at 100 ◦C under air (see
runs H1–H5 in Table 2). Fig. 2 compares the conversion curves of PhI
in these runs. Though quantitative conversion of PhI was obtained
with all five catalysts (turn-over number, TON = 10,000), signifi-
cant differences in their turn-over frequency (TOF) and conversion
curves can be noted. The three heterogeneous catalysts (HBPPA-Pd-
1 to HBPPA-Pd-3) synthesized with PPh3-ligated L1-Pd (TOF = 2000,
2500, and 1700 h−1, respectively) were clearly much more active
with significantly shortened reaction time when compared to
HBPPA-Pd-4 (TOF = 500 h−1) and HBPPA-Pd-5 (TOF = 400 h−1). With
HBPPA-Pd-2, quantitative conversion was reached at as short as
4 h while it took 20 and 28 h with HBPPA-Pd-4 and HBPPA-Pd-5,
respectively. Meanwhile, in the reactions with the latter two, a
long initial induction period (ca. 6 and 4 h, respectively) with no
PhI conversion was noticed (see Fig. 2).
Given their similar Pd content, the dramatic difference in
the catalytic performance of these three heterogeneous catalysts
reflects the significant effect of the ligand of the encapsulated Pd
species on the reaction. The PPh3 ligand is a commonly used clas-
sical ligand for Pd catalysts in many cross-coupling reactions [64].
Several diphosphine ligands in combination with Pd(OAc)2 have
been reported to efficiently catalyze the Mizoroki–Heck reactions
of aryl chlorides [83–86]. Meanwhile, a range of diimine ligands
in combination with Pd(OAc)2 or Pd(acetylacetonate)2 have also
been shown to effectively facilitate both the Mizoroki–Heck and
Suzuki–Miyaura reactions [87,88]. On the basis of the catalytic per-
formance of these three heterogeneous catalysts, PPh3 is far more
3.3. Suzuki–Miyaura reactions catalyzed with HBPPA-Pd-2
Suzuki–Miyaura reactions of various aryl/heteroaryl bromides
and aryl chlorides with arylboronic acids were carried out with
HBPPA-Pd-2 in water as the environmentally benign solvent under
air at 100 ◦C. Table 3 summarizes all the reactions. In the case
with the relatively more active aryl bromides, reactions were
performed with K2CO3 as the base without the use of any phase
transfer agent. HBPPA-Pd-2 shows outstanding catalytic activity
toward these reactions. In reactions employing more difficult aryl
chlorides, tetra-n-butylammonium bromide (TBAB) was used as
the phase transfer agent, along with CsF as the base, in order to
enhance catalyst activity. In runs S1–S3, the reactions between
PhBr and phenylboronic acid (PBA) were carried out at different Pd
loadings (10, 1, 0.3 mol ppm, respectively). Quantitative conversion
of PhBr was successfully achieved even in run S3 undertaken at
the lowest Pd loading of 0.3 mol ppm, rendering the highest TON
(3,200,000) and TOF (188,000 h−1). To the best of our knowledge,
these activity values are the highest compared to those achieved
in the literature with heterogeneous Pd catalysts under similar
conditions. The best results in the literature (TON = 3,570,000;
TOF: 119,000 h−1) were achieved by Uozumi et al. in the reaction
between aryl iodotoluene and PBA with their self-assembled

68
Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
heterogeneous poly(imidazole-Pd) catalyst at the Pd loading of
0.28 mol ppm [46,47]. However, their results were achieved with
the use of relatively more reactive aryl iodide in the presence a
phase transfer agent that often improves catalyst activity.
100
80
60
40
20
0
3.5
3
2.5
2
1.5
1
0.5
0
conversion
leached Pd
At the Pd loading of 10 mol ppm, the reactions of various aryl
bromides containing different substituting groups with PBA (runs
S4–S9 in Table 3) were successfully accomplished with quan-
titative conversions achieved. The TOF values are high, ranging
from about 10,000 h−1for deactivated aryl bromides (methyl-
substituted phenyl bromides and 4-bromoanisole in runs S6–S9) to
100,000 h−1 for activated aryl bromides (4-bromobenzonitrile and
4ꢀ-bromoacetophenone in runs S4 and S5). Moreover, the reactions
(S10 and S11) involving sterically hindering 1-bromonaphthalene
and 2-bromomesitylene were also successfully accomplished with
high conversions but at a slightly higher Pd loading of 50 mol ppm.
Similarly, quantitative conversions were also achieved in the
reactions of aryl bromides with activated or deactivated substi-
tuted phenylboronic acids (runs S12–S15) at the Pd loading of
10 mol ppm.
1
2
3
4
5
6
Cycle of Reaction
Fig. 3. PhBr conversion and percentage of leached Pd in the reusability test of
HBPPA-Pd-2 in the Suzuki–Miyaura reaction between PhBr and PBA. Other reaction
conditions: [PhBr] = 0.6 mol/L, [PhBr]:[PBA]:[K2CO3] = 1:1.3:1.5, Pd loading relative
to PhBr = 0.1 mol%.
with a CA conversion of 64% reached after 48 h of reaction, giving
high TON (640,000) and TOF (13,000 h−1) values. In the reactions
with less reactive PrA (runs A6 and A7), quantitative conversions
were reached within 24 h at the Pd loading of 50 mol ppm. These
reactions thus confirm the high performance of HBPPA-Pd-2 in
facilitating the allylic arylation reactions.
We subsequently investigated the performance of HBPPA-Pd-
2 in the heteroaryl bromide-involving Suzuki–Miyaura reactions,
which are of vital importance in medicinal/agrochemical fields [90].
There are quite a few homogeneous Pd catalysts reported to be
capable of catalyzing versatile Suzuki–Miyaura reactions of het-
eroaryl compounds with high Pd loadings (1 mol% or even higher)
[91–101]. However, only very limited heterogeneous Pd catalysts
have been developed for such reactions [53,64,102–106]. Herein,
we have found that HBPPA-Pd-2 is also efficient in catalyzing the
reactions of representative heteroaryl bromide (2-bromopyridine
and 3-bromothianaphthene) with PBA. Due to the lower activity of
such compounds, a much higher Pd loading of 5000 ppm was, how-
ever, needed, and the solvent was also changed to ethanol/water
mixture with K3PO4 as the base. As shown in Table 3 (S16 and S17),
quantitative or nearly quantitative conversions were achieved in
3.5. Reusability of HBPPA-Pd and Pd leaching during
cross-coupling reactions
Relative to the homogeneous catalysts, heterogeneous catalysts
show the advantage in their easy recovery and reusability. To con-
firm the reusability of HBPPA-Pd-2 as a reusable heterogeneous
catalyst, we carried out a six-cycle reusability test of HBPPA-Pd-2
in the Suzuki–Miyaura reaction between PhBr and PBA at the high
Pd loading of 1000 mol ppm at 100 ◦C. After the first cycle of reac-
tion, the same catalyst was recovered and reused for additional 5
times. Each cycle of reaction lasted for 3 h. At the end of each cycle of
reaction, the heterogeneous catalyst was recovered easily through
filtration, washed, and dried before being used for next cycle of
reaction. The reaction solution in each cycle was collected and ana-
lyzed for the conversion of PhBr and the concentration of leached
Pd species. Fig. 3 summarizes the conversion of PhBr and the per-
centage of leached Pd species in each cycle. Quantitative conversion
was maintained in the first 4 cycles of reaction, but the conversion
dropped slightly to 81% in the 6th cycle of reaction. From AA mea-
surements, the filtered reaction solution in each cycle contained
leached Pd species at a concentration of around 2 mg/L (i.e., 2 ppm),
indicating that about 2.8% of encapsulated Pd leached out during
the reaction (see Fig. 3). In all 6 cycles of reactions, the total percent-
age of leached Pd is about 16.3%, which should still be considered
low. Overall, this test indicates that the heterogeneous catalyst has
good reusability and stability.
such reactions, resulting a TON around 200 and TOF around 10 h−1
.
This result ranks HBPPA-Pd within the very limited heterogeneous
Pd catalysts that have been reported to facilitate Suzuki–Miyaura
reactions of heteroaryl compounds [53,64,102–106].
Compared to aryl iodides and bromides, aryl chlorides are much
more difficult to activate in cross-coupling reactions but highly
desired due to their abundance and low cost. The coupling reac-
tions of aryl chlorides with the use of heterogeneous catalysts are
often challenging [107]. We find that HBPPA-Pd-2 is effective in
catalyzing the reactions of various aryl chlorides with PBA at the
Pd loading of 1000 mol ppm (see runs S18–S22). The highest TOF
value of 130 h−1 was achieved with the activated aryl chlorides
(4-chlorobenzonitrile and 4ꢀ-chloroacetophenone in runs S18 and
S19). All these results confirm that HBPPA-Pd-2 is highly active in
catalyzing the Suzuki–Miyaura reactions.
3.4. Allylic arylation reactions catalyzed with HBPPA-Pd-2
We have further investigated the performance of HBPPA-Pd-2
in catalyzing the allylic arylation reactions. Compared to aryl-aryl
coupling, allylic arylation (i.e., allyl-aryl coupling) is less studied yet
more challenging and often requires a large amount (1–10 mol%)
of catalyst [108–112], except few heterogeneous catalysts that can
catalyze allylic arylation at ppm level of Pd [46,47,64]. Herein, the
allylic-arylation reactions between two typical allylic acetates (cin-
namyl acetate, CA, and prenyl acetate, PrA) and tetraphenylborate
(NaBPh4)/PBA were carried out with HBPPA-Pd-2 in water/DMF
mixture at 100 ◦C under air. Table 4 summarizes these reactions.
In the reactions (runs A1–A5) between CA and NaBPh4/PBA, quan-
titative conversions of CA were reached at the low Pd loadings of
10 and 50 mol ppm. At the further reduced Pd loading of 1 mol ppm
(run A3), the reaction between CA and NaBPh4 still proceeded
Given the presence of low but appreciable Pd leaching, we fur-
ther carried out a hot filtration test to investigate the catalytic
mechanism. In many heterogeneous catalyst systems with sig-
nificant catalyst leaching, it has been found that the leached-out
homogeneous Pd species present in the solution are the ones effect-
ing the coupling reactions instead of the heterogeneous Pd species
[39,67,113–116]. In the test, the reaction solution of PhBr and PBA
with HBPPA-Pd-2 at the Pd loading of 1000 mol ppm was hot fil-
tered after 0.5 h of reaction at 100 ◦C (PhBr conversion = 71%). The
filtered solution was continued for the reaction at 100 ◦C. There is
negligible increase of PhBr conversion in the filtered solution during
the first 8 h (from 71% to 72%). Then the conversion of PhBr showed
a gradual increase to 81% after 20 h and remained unchanged after-
ward despite being extended for another 20 h. On the opposite, the

Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
69
Scheme 2. Synthesis of heterogeneous Si-ArI and its possible reaction in 3-phase test.
control run undertaken at identical conditions but without involv-
100
80
60
40
20
0
ing hot filtration was completed within 1 h of reaction with no
induction period observed. See Fig. 4 for a comparison of their con-
version curves. From this hot filtration test, it can be concluded that
the leached-out homogeneous Pd species still have some activity,
but considering the long induction period and the slow reaction
speed compared with the heterogeneous catalyst, they have only
small negligible contributions toward the reaction and the hetero-
geneous encapsulated Pd species play the key role [39,67,113–116].
Besides the hot-filtration test, the 3-phase test, which entails
anchoring one of the reactants onto a solid support, is an alterna-
tive important technique in the determination of heterogeneity in
cross-coupling reactions facilitated with solid-supported Pd cat-
alysts [67,113,116–118]. If the cross-coupling reactivity results
completely from the heterogeneous Pd species embedded in the
solid support, no transformation should be observed with the
anchored reactant. On the other hand, if the leached homo-
geneous Pd species have some activity, the anchored reactant
will be converted to some degree. As shown in Scheme 2,
we designed a 3-phase test for the Mizoroki–Heck reaction
conversion of homogeneous COMePhI
conversion of heterogeneous Si-PhI
0
4
8
12
16
20
24
Fig. 5. Conversion plots of homogeneous (COMePhI) and heterogeneous (Si-
PhI) aryl iodides in the 3-phase test. Reaction conditions: [ArI] = 0.6 mol/L,
[ArI]:[BA]:[Et3N] = 1:1.3:1.5, Pd loading relative to [ArI] = 0.1 mol%, DMF as solvent
at 100 ◦C.
Similar to the hot-filtration test, the reaction was also undertaken
at very high Pd loading (1000 mol ppm relative to COMePhI).
As shown in Fig. 5, no appreciable transformation of hetero-
geneous aryl iodide was achieved after 2 h of reaction, whereas
the conversion the homogeneous COMePhI reached a high con-
version of 70%. Subsequently, the homogeneous aryl iodide was
quantitatively converted in 4 h. But even after extended reaction
time of 24 h, the conversion of heterogeneous Si-PhI is only 40%.
Considering that most cross-coupling reactions in this work were
performed at Pd loadings less than 1000 mol ppm and the reac-
tion time were around or less than 24 h, the 3-phase test further
supports that HBPPA-Pd-2 catalyst is a heterogeneous catalyst and
the leached homogeneous Pd species only have a small contribu-
tion compared to the immobilized Pd species. This differentiates
HBPPA-Pd-2 catalyst from many other reported heterogeneous Pd
catalysts where the leached homogeneous Pd species are in fact the
true ones responsible for the reactions [39,67,113–118].
in organic solvent, with the use of
a mesoporous silica-
supported aryl iodide (Si-ArI) as a part of the aryl halide [64].
In addition, 4-iodoacetophenone (COMePhI), which has simi-
lar reactivity relative to the heterogeneous counterpart, was
used as the homogeneous aryl halide (Si-ArI:COMePhI = 0.1).
100
control run
80
hotfiltration
60
40
20
0
4. Conclusions
0
10
20
30
40
We have demonstrated in this work the synthesis of hetero-
geneous Pd catalysts for coupling reactions with the use of a
HBPPA containing pendant alkyne groups as the cross-linkable
polymer substrate via one-step encapsulation of the Pd species
that catalyze the cross-linking reactions. The in situ generated
triphenylphosphine-ligated Pd(II) catalyst (L2-Pd) has been
reaction time (h)
Fig. 4. Conversion curves of the Suzuki–Miyaura reaction between PhBr and PBA
involving hot filtration and the control run without hot filtration. Reaction con-
ditions: [PhBr] = 0.6 mol/L, [PhBr]:[PBA]:[K2CO3] = 1:1.3:1.5, Pd loading relative to
[PhBr] = 0.1 mol%, water as solvent at 100 ◦C.

70
Z. Dong, Z. Ye / Applied Catalysis A: General 489 (2015) 61–71
identified to be the most optimum one for effecting the cross-
linking and encapsulation. We have found that HBPPA-Pd-2 is
efficient and highly active in facilitating the Mizoroki–Heck,
Suzuki–Miyaura, and allylic arylation reactions with the relative
molar Pd loading at ppm levels. A high TON of 86,000 and a high
TOF of 1800 h−1 have been achieved in the Mizoroki–Heck reaction
between PhI and BA with HBPPA-Pd-2 at the low Pd loading of
10 mol ppm. More notably, HBPPA-Pd-2 showed very high activity
in Suzuki–Miyaura reactions with various aryl/heteroaryl bromides
(Pd loading down to 10 mol ppm with TON up to 100,000 and TOF
up to 100,000 h−1) and low-reactivity aryl chlorides (at the Pd load-
ing of 1000 mol ppm with TON = 1000 and TOF up to 130 h−1). In
particular, the reaction between PhBr and PBA occurred quantita-
tively even at a very low Pd loading of 0.3 ppm, rendering very high
TON (3200,000) and TOF (188,000 h−1) values that are comparable
to or even better than the best values reported in the literature.
The allylic arylation reactions were also successfully accomplished
with HBPPA-Pd-2 at the Pd loading of as low as 1 mol ppm (TON
up to 640,000 and TOF up to 13,000 h−1). The 6-cycle reusability
test of HBPPA-Pd-2 in the Suzuki–Miyaura reaction between PhBr
and PBA has confirmed that the heterogeneous catalyst is easily
recyclable and reusable with well-retained catalytic activity and
low Pd leaching. A hot filtration test, together with a 3-phase test,
has also verified that the catalyst is truly a heterogeneous one
with negligible contributions from the leached homogeneous Pd
species toward the catalytic activity. The heterogeneous catalyst
synthesized through this unique simultaneous cross-linking and
encapsulation strategy thus has the good potential for applications
in carbon–carbon cross-coupling reactions.
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Acknowledgements
The financial support from the Natural Sciences and Engineer-
ing Research Council (NSERC) of Canada and Canada Research Chair
(CRC) is greatly appreciated. ZD thanks the Ontario Ministry of Eco-
nomic Development and Innovation for awarding a postdoctoral
fellowship.
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